Tomorrow I’m travelling over to Leeds to attend a seminar organised by the Chemical Industries Association seminar on Controlling and Measuring Occupational Exposure to sulphuric acid mist. I’ll be making a brief presentation on sampling methodologies. I’ve uploaded a copy of the presentation to my Slideshare site, and it can be viewed below.
Exposure to sulphuric acid mist can occur in a number of industries including during sulphuric acid manufacture, loading and unloading tankers, transfer between storage vessels, charging lead acid batteries, pickling and plating operations and other processes.
A new Workplace Exposure Limit (WEL) for sulphuric acid mist came into force in December last year when the UK implemented the latest batch of European indicative occupational exposure limit values (IOELVs). The most notable feature about this WEL, besides having a low value of 0.05 mg/m3, the limit refers to the thoracic fraction. This is the first thoracic limit that has been set, all other particulate WELs being for either the inhalable or respirable fractions.
It is questionable whether a thoracic limit is appropriate for sulphuric acid. The main concern, besides upper respiratory tract irritation, is the potential for tumour formation in the respiratory tract, believed to be a consequence of sustained tissue inflammation and repair processes, with the larynx the main site of concern. As the thoracic fraction excludes particles that don’t penetrate beyond the larynx, it doesn’t really seem appropriate for a limit to be set that excludes the main particles of concern. The original recommendation from SCOEL (the European Union’s Scientific Committee on Occupational Exposure Limits) was for a limit based on the inhalable fraction, but by the time it reached the EU Council it had become a thoracic limit and that’s what ended up in the EU directive and, ultimately, as a UK WEL.
The difficulty the thoracic limit presents is that, currently, we don’t have a validated method for measuring the thoracic fraction of the mist. There are some options, but work needs to be done to determine which method is most suitable and to properly validate it. Given the current political climate with major cutbacks in expenditure by the Health and Safety Executive, I think it’s highly unlikely that they’ll fund this.
The easiest approach is to carry out sampling for the inhalable fraction. If the results are below 0.05 mg/m3, then as the thoracic is a sub fraction of the inhalable convention, then they would demonstrate compliance with the WEL. If the results are substantially greater than 0.05 mg/m3, then there’s a good chance that the WEL will have been exceeded and additional controls need to be introduced. However it would be difficult to interpret results that are only just higher than 0.05 mg/m3.
European Commission, Employment, Social Affairs and Inclusion, January 2007 Recommendation from the Scientific Committee on Occupational Exposure Limits for sulphuric acid SCOEL/SUM/105
- Health and Safety Executive, 2011, EH40/2005 Workplace Exposure Limits, Second Edition